As an extension of previous experiments attempting to characterize the optimal laser source and transmitting optical fiber for intravascular precise microablative surgery, we have been working with the Naval Research Laboratory utilizing a prototype Erbium (Er):YAG laser with developmental zirconium fluoride fibers in necropsy human and animal tissues. The Er:YAG laser operates in the infrared (2.9 nm) and all of the energy is absorbed within a 10 micron zone of tissue, due to the very high absorption coefficient of water at this wavelength. Studies in air and through fibers in a wet field demonstrated histologic effects resulting in precise triangular crater formation without significant surrounding thermal tissue injury similar to previous work done with Excimer lasers in our laboratories. The ablative threshold for tissue using the Er:YAG laser and optical fiber system was comparable to KrF (248mm) whereas the fiber damage threshold was greater than 400 mJ/mm2. In addition, this laser-fiber combination was capable of easily ablating calcified tissue (including bone), albeit at much higher ablative thresholds and lower ablative efficiencies. We believe that this new infrared laser-fiber system is highly suited for intravascular work and offers several advantages compared with Excimer lasers. These include 1) a solid state more compact reliable system design, 2) no ultraviolet radiation or gas hazards, 3) similar precise histology effects, and 4) delivery in vivo through smaller, more flexible fibers with a more favorable overall energy density operating range.